Cytomegalovirus (CMV) is a major cause of morbidity and mortality in the immunocompromised host, especially in those having undergone solid organ or bone marrow transplants. A simple and sensitive method for the early detection of CMV blood infection in these patients would be helpful for the medical management of the patient. Efforts will be made to determine the presence of CMV~antigen in blood by detecting the pp65 lower matrix structural protein of CMV in polymorphonuclear leukocytes. Detection of the antigen with monoclonal antibodies may be useful as an early predictor of CMV infection. Comparisons of the sensitivity, specificity, speed, and ease of determining CMV antigenemia will be made with the shell vial viral culture and with a polymerase chain reaction assay utilizing the immediate early gene region of CMV for primer recognition.